Sea-bottom organisms (Benthos)
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Organisms that live on the seafloor surface or in the sediment (mud and sand) are called benthos. They include organisms from diverse taxonomic groups such as seaweed, sea anemones, corals, shellfish, gobies, starfish, and flatfish.
Benthos can be collected by dragging a net or bucket across the seafloor or by using a mud sampler to collect the entire sediment that is their habitat. The efficiency of collecting organisms with a mud sampler depends on the hardness and characteristics of the bottom sediment (e.g., sediment or gravel), but compared to the method using a net, it is possible to obtain samples of organisms living below the seafloor surface, i.e., in the sediment.
Collecting large benthic fishes requires the use of large commercial fishing gear, such as otter trawls, which require extensive specialized fishing equipment and experience. The collection gear used for research is designed to be used by research vessels without fishing equipment.
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【Towing type collector】
Benthic organisms are collected by dragging a net or bucket over the seafloor. The collection gear is selected according to the community to be sampled, taking into consideration the towing speed and the size of the net opening. If the population density is high and the organisms can be collected by simply scraping the seafloor surface, they are collected using a dredge, as shown in Fig. 7. On the other hand, if the population density is low and a larger seafloor surface needs to be scraped, or if benthic fish with swimming ability need to be collected, a "sledge net", a kind of the beam trawls (Fig. 8) is effective.
Fig. 7 Dredge
By scraping the bottom of the seafloor, organisms that live on the seafloor surface and in the bottom sediment are collected.
a) organisms dredge b) cylinder dredge
Fig. 8 Sledge net
A type of beam trawl with sledge shape frames on both sides of the net opening. The net is towed by sliding the frames across the seafloor. Since the width of the net opening is constant, the area of the towed net can be easily calculated by adding up the towing distance. The population density can be estimated by dividing the number of individuals collected by the area of the towed net.
ビーム: beam 内網: inner net 外網: outer net ソリ状フレーム: sledge shape frame
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【Bottom sampler】
Organisms that live on or under the seafloor are collected with the seafloor sediment where they live. Unlike methods using nets, samples do not fall through the mesh, so even small organisms are collected. Since the entire habitat of the organisms is sampled, the possibility of live capture is high and damage to the biological sample is minimized. There are two types of mud samplers: those that grab the sediment with a bucket (grab type bottom samplers), and those that stick a pipe into the seafloor and pull it out while maintaining the stratigraphy of the sediment (column shaped corers).
Fig. 9 Smith-McIntyre Bottom-sampler
This is a type of grab type bottom sampler that collects sediments by grabbing them with a bucket. When the bucket touches the bottom, the trigger is activated, and the bucket is thrust into the seafloor by the spring force. When the bucket is pulled out, it closes and grabs the benthic organisms along with the sediment.
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